Last turn, player A casted a Pact spell, and then a copy of Mindslaver's ability, controlled by player B, resolved while targeting player A. In player A's untap step, player A untapped 5 basic lands of the type that corresponds to the Pact spell's color. Tapping those lands for mana is the only way to tap those lands, and paying for the Pact spell's delayed triggered ability is the only way for player A to spend the mana which those lands can produce.
Can player B nonetheless choose to have player A not pay for that triggered ability?
The Pact spells do not make the payment optional, but there is precedent for players not being forced to pay costs. (See the two sentences with the word "forced" in that link.)