You may wonder: CAN an infinite value be taken from an infinite quantity and still leave an infinity quantity? The answer is yes. For instance, the cardinality of the set of natural numbers is infinite. Remove from this set the infinite elements which are even, and you are left with the infinite set of odd positive numbers.
You may wonder: MUST an infinite value, when taken from an infinite value, always yield an infinite value? The answer is no. For example, take the set of natural numbers, a set with infinite cardinality. Remove from this the set of natural numbers, a set with infinite cardinality. The remainder is an empty set, with 0 elements.
Conclusion: When substracting infinity from infinity, you can, depending on how you define subtraction and your infinities, wind up with infinity or a finite value.