If a balloon payment becomes due on a date that falls on the weekend, is it extended to the next business day?

Depends on what is listed in the contract. Common usage is yes. But there is no law on it. So I would not risk guessing. Contact the other party in the contract and make sure. If you will have the money on Monday, then you should have it on Friday.

It will depend on wording in contract as Scott said If it was my contract, then no, the day due, is the day due. Buyer is just required to have payment in prior to that date.

Also, if the payment is mailed, it should be postmarked on or prior to the due date. If it will be hand delivered then you should check as I said.