Problem is, in most properly "medieval" societies, money didn't exist as we think of it now. Coins existed, but were simply a more-or-less convenient way of handling gold. Gold was a tradable commodity just like grain, swords, or whatever--coins and other units were originally just a way of measuring and assaying gold. In other words, a gold coin was simply a lump of gold which you could assume was X weight and Y purity.
Of course, this is where clipping coins and other forms of counterfeit and theft come in. If you could shave a bit of gold off of every coin that crossed your hands, without others discovering their gold coins had been 'lightened,' you could make a profit. Once again, though, you end up with coins being worth no more than their weight in gold (or silver, et al).
In this sense, a 'pawnbroker' isn't any different than any other merchant--trading your loot for gold is no different than trading it for some other good. Now, the moneylender is something completely different, and exceedingly common.
When they were finally introduced (and this is one of the distinctions that helps separate Medieval from Renaissance societies), letters of credit or bills of sale were indeed a form of credit, or money-notes. However, you wouldn't give them to just anybody. Treating slips of paper as though they had value took a long time to develop.
Of course, D&D is basically a modern world with medieval flavor--you have cash societies, mass-produced weapons (and potions of Cure Light Wounds), long-distance trade, and so on.
The short answer? Stop giving out thousands of coins as reward. Give them artifacts, art objects, jewels, fine clothes*, and so on. Since "medieval banking" didn't really exist--not in the sense you're thinking of--instead it becomes a question of whether you want to insert another modern-world anachronism into your D&D setting, or force the characters to do without the service in the name of 'realism.'
I've actually been thinking one of the interesting things about doing some sort of 'ancient world' RPG, would be that coin money would be mostly nonexistent....so every time you bought or traded something, you would be bartering for everything, instead of 'purchasing' it. "I'll give you this dagger and a cask of beer if you repair this armor for me..."
* Clothes are definitely undervalued in D&D. Before sewing machines were invented, fine clothing was one of the most expensive items you could buy--because each item represented massive quantities of hand stitching. Not to mention the costs of importing fine cloth from far away. Rich women wearing ten layers of skirts and petticoats in the 1700s didn't come from some weird sense of modesty--it was conspicuous consumption. Each additional layer of clothing represented the wealth it took to pay someone to hand-stitch it for you....
, but I can see this as operation of mutual interest.
