Make sure when they put it in writting that the settlement letter-offer is on their letterhead, that the account is referenced (account number as well as the original creditor) and that the person in the signature block is authorized to make the settlement.
Also, make sure the letter states the who, what, when, where and how:
Who: Them and You
What: Account Info and Settlement Offer
When: The Payment Agreement (post-dated check)
Where: to send the funds and
How: to send the funds (FedEx?)
Also, I would make sure that they include in the letter that they will mark the account as Settled in Full and zero out the balance in your credit file. If the account is getting a settlement offer, then it is most likely far behind in payment. So, the damage to your credit is done. If the account was closed due to nonpayment, then your balance-to-credit ratio was upside-down (meaning you owe an amount greater than your available credit on that account). If the account went into charge-off, then the worst (besides bankruptcy) mark is already on your account. So, zeroing out the account may actual help... how much it helps depends on a lot of factors.)