Extension to pay court fine

Technically speaking, if they're going to try to violate you on fines only, they have to prove the non-payment was "willful" (voluntary conscious and intentional). The absolute best way to show your non-payment is not willful is to pay something every month through the duration of your probation. Since that boat has long since sailed, the way to best protect yourself is to pay as much as you possibly can each of the next few months.

Judges usually give extra time but that also means your Probation will be extended.

You need to make every effort to pay your fine in full by the deadline. However, if you show a good faith effort, courts generally will allow an extension. $640 of $3,000 in about 21 months does not seem like you are working very hard.