A ball 2/8 at the 50. A36 runs to B's 40 and fumbles. During the loose ball B25 grabs and twists A41's face mask. B20 recovers the ball at the B 39 yard line. What do we have?

This is the good 'ol clean hands rule. It's going to be A's ball 1st and 10 at B's 25 after the 15 yard penalty is marked from the end of the run (B's 40). B can't retain possession if they foul prior to gaining that posession, unless A inexplicably declines the penalty.

NCAA: basic spot foul (end of related run) enforcement, which means A's ball, first and 10 from the B 25.