Hang on here - nobody's asked the vital question:
Might there be a reason you can't convert ritual scrolls into ritual books?
And yes there is - ritual scrolls as written leave the control (the flow of ritual casting) up to the DM. If you want to lend the players a hand in some adventure, you might arrange for them to find or buy up to three ritual scrolls. This allows them to cast that ritual (for example; finding their way through a labyrinth) three times but not more.
Once you've given the adventurers a ritual book, they can effectively cast it forever (barring minor issues such as component costs). For the purposes of my example adventure, this might ruin it completely (if they cast the pathfinding ritual at every turn in the maze, the entire exercise becomes pointless).
Same thing, another example: you could have your heroic level party find a single scroll of Raise Dead. This essentially gives them a single "get out of jail" card, but they can only mess up once, keeping them on their toes. Otherwise, you'd immediately end up with a situation where level means everything (are they level 8? Everybody gets as many "get out of jail" cards as they want. Are they level 7? No they don't) and such a sharp delineation between levels 7 and 8 are most probably
not intended.
So the inability to convert a consumable into an at-will (as it were) is probably intended to be a
positive quality of the game, and not a detriment.
In short; I feel it is much better you tell your players you have someone willing to trade in the scroll as part of the payment for a book containing the same ritual just this once.
The important part is "just this once". You wouldn't want to give up all future control just because your in this particular situation now.
Zapp
PS. If you want, you could re-skin scrolls into glowing gems of crystallized magic. This I feel makes for a less strong connection than between book and scroll: it becomes much less "reasonable" that you could convert one into the other.