If the DM is doing his job right, there will indeed be a "Gotcha" in the mix.
My groups have come to fear such opportunities. The Devil's deal always sounds reasonable, even advantageous for the party to the point of the Devil being overly generous.
Presume that something will go wrong, and that the Devil will know it in advance.
Now, there is an obvious advantage for the Devil to have the Abyssal portal closed. The "Blood War" between Hell and the Abyss is a long running one, and if the Abyssal portal is closed and the Infernal one left open, then Hell has a permanent toehold on that world.
The less obvious consideration is that the Abyss can use the portals as a way to invade the Nine Hells directly. They have the greater numbers, after all, so the link is the advantage of the Abyss. So the Devil *really* wants one portal or the other closed as soon as possible.
Now if the Devil has the ritual to do that, you have to ask yourself, why isn't he using it? Yeah, it's great to manipulate mortals into taking all the risks, and if you can get them to pay for the privilege of taking all the risks, so much the sweeter.
But while the possession of that ritual gives him an apparent edge at the bargaining table, that edge is an illusion, a bluff. Yes, this mortal realm is at risk, so you need the ritual. But his realm is at risk as well, and his need at least as great as yours. He's hoping that you don't realize that. If you do, he doesn't get to have you pay him for the ritual. Instead he might end up having to pay you to perform it, because if he could, he would have already.
So don't be afraid to bargain hard when the time comes. And remember, even if you "win" at the bargaining table, there will be (or should be) an unforeseen price in the end.
<Story/tangent>
In one game I ran, the party needed information from a Devil lord. He was a gracious host when they came to see him, offering his promise of safe passage to and from his palace, and hat the rules of hospitality would apply during their visit: They would be treated as guests so long as the behaved as guests.
At dinner, he had brought in a number of lovely ladies, and he explained that sometimes people get sent to him who really didn't belong in Hell. Somebody sold a romantic rival or unwanted servant or slave, or some adventurer came down and couldn't get away. These ladies were all in that category, and what was more, all were still alive. The great heroes were given their choice of companions for the night. They were to "do their heroic best" (do I need to get specific?) with the girls. Each girl one of the heroes could bed, in a single night, would be afforded comfortable lodging and cre for the next year, with no torment provided. At the end of that time, if the party had used the information as promised, the ladies would be released back to the mortal realm, and provided for to a small extent.
The party Paladin was of course suspicious. He didn't want to bargain with the Devil in the first place, of course. He caught the catch in the deal: What of any children that resulted from the night's festivities? The Devil pointed out that the deal was only for the ladies. Any other souls released would be a separate deal.
The Devil wanted mortal agents, children of great heroes, raised in Hell, that he could let loose upon the world. "Devils" inthe world who weren't "outsiders", and couldn't be banished, or even easily traced.
One of my favorite parts of the scene was when someone asked about the female PCS. The Devil, ever generous, offered them as many innocent young men as they wanted. The men would be released immediately, and the female PCs would be afforded comfort in Hell for a year, then be allowed to leave.
"That's not fair!", cried the Paladin.
The Devil's response? "I don't know who has been spreading this tale that there's anything fair going on in Hell, but I'd really like to thank them! It makes these meetings so much more fun."
So a night of revelry for the PCs, with partners who are more than willing to participate, partners who truly don't belong there, and a chance to free them and return them home. Oh, and in addition, the Devil would give them the information they needed.
And just the one poison pill that made the entire deal unpalatable.
</story>
The point of the tale was that, while the Devil (being Lawful Evil) may make it a point of honor not to actually lie in the making of a bargain, if run properly he should always have not only an ace up his sleeve, but probably several extra sleeves as well.