I'm assuming you heeded the advice here from other 'mainers and you won't give him the name til he's paid in full. I think the only way I'd entertain pymt by installments is if the offer was really good and the buyer asked me to let him use the domain while he pays (direct the domain to wherever he wants) If the offer is nothing out of the ordinary then I don't understand why anyone would take pymt by installments rather than telling him to pay you when he has all the money, and if the domain is still available (if a better offer comes around he can match it but I wouldn't hold the name for him)
A couple of problems with what he's suggesting.... 1) Will he pay for a lawyer to prepare a contract, and even if he does you need someone on your side to make sure you're protected, in most cases ... 2) Once you redirect the servers to him, you need to constantly make sure he won't get you in trouble (hate speech, extreme porn, watch what he does with the domain lol....) because the WHOIS will still list you as the registrant ..... 3) Two yrs is an awfully long period, I don't like this but it's up to you .... 4) I'm sure you'll keep your end of the agreement, you're bound particularly if you live in the same country, but how would you feel if 10 mos from now a guy contacts you thru the WHOIS and makes you a better offer ? It's easier to sell in one shot for that reason, those are the so-called seller regrets
Written agreements and domaining aren't usual combinations, they're rare, unnecessary IMO and even obsolete. Having said that, you are on the preferred side here since if he stops paying you won't give him the name, he's actually taking more risk since you could try to weasel out the deal (in theory, I know you won't) and technically you could take his money and deny him the domain. I suppose another option is giving him your word you'll sell to him in 2 yrs (no pymt in installments) UNLESS you get a better offer during those 2 yrs, and if you do you'll have to sell to the other guy if he doesn't match it. And take a look of what may go wrong with "written agreements", I know it's the reverse in that case (link below), the aggrieved party is the buyer who paid in full and a scamming seller didn't give him the domain, and he didn't return his money either.... but in theory it's a good example. Domain leasing seems to be en vogue right now, and there some type of contract is needed, you might look into that too (lease first and they buy it when he has the $, getting credit for the $ spent leasing)
http://www.dnforum.com/f77/renegged-domain-sale-offer-after-receipt-payment-thread-505272.html