Can seller dictate lender?

[Nancy Bachety]

Contract is signed by us, buyer. Seller signed after seller's agent added a condition, as follows, that we were told to sign. 

"Buyer is required to use Joe Lender". 

We have a lender lined up who provided a preapproval. We were told seller and his agent want us to use a local lender. We sought and provided another preapproval from Joe lender, local, to comply.

Now, seller signed the contract, with that requirement. 

We and our agent, believe that's not legal. Do we sign, knowing it's illegal? Our lender is one we've used several times and has superior terms.

Do we sign, and not worry?

Deal is in FL.