What Matt didn't know was that Ben had lied. There was no time limit, Ben could've easily turned Matt back to normal.
But Ben wanted more. He'd always liked Matt, but never thought that he would actually have a chance.
When Matt asked Ben to use him, Ben had been overjoyed.
Matt being willing to be that intimate with Ben meant that there had to at least be some attraction.
After Matt hadn't contacted Ben for a few days Ben became nervous. Had they moved too fast?
Could this endanger their friendship? He came to visit Matt to talk with him about what happened,
maybe they could even become more than Friends. After that, when he'd found Matt and was about to turn him back, Ben hesitated.
He could have made Matt love him, make himself the only thing Matt would ever care about.
It would have only taken him a few clicks to get everything he ever wanted.
But he couldn't.
He loved him and didn't want Matt to only have feelings for him because a machine forced him to.
Ben wanted Matt to love him for who he is. But he didn't know how to do it.
Then Ben had an idea. What if Matt had to rely on him in some way?
What if he could give Matt a reason to come to him, maybe even be intimate with him?
Ben was sure that he could win Matt over if he became someone Matt relied on.
That's why he lied to Matt.