If you are referring to a credit card or other type of loan known as open-end credit, your available credit limit typically increases when the payment is posted to your account. However, the decision of when to replenish the credit limit is up to the credit union and, in some circumstances, a credit union may delay replenishing a credit line. If the credit union delays replenishing a credit line, it cannot charge an over-the-limit fee even if the consumer has opted in to allow overlimit fees.

If you are referring to a closed-end credit arrangement, then your payments simply reduce the outstanding balance—with no ability to charge other amounts to it. 
To learn more about your particular account, consult with your lender.