Yes. You should have an executed agreement detailing everything you discussed re payment plan BEFORE making any payments. And stick to it to the letter. Otherwise, everything will be exactly as it was before the agreement, interest penalties, etc.
You should have a settlement agreement signed by all parties prior to making any payments. If the payment is due soon, contact the other party and tell them that you need the agreement signed and until it is signed the payment will not be made. A digital copy can be acceptable as an "original" signature which might help with the pending deadline.