How do casinos split uneven bet?
From Wikipedia:


  $2.50 chips are mostly used for blackjacktables, since a "natural" (a 21 on the first two cards dealt to a player) typically pays 3:2 and most wagers are in increments of $5. However, the Tropicana Casino and Borgatain Atlantic City, New Jersey, and others, have used $2.50 (pink) chips in $7.50 to $15 and $10 to $20 poker games.


They have $2.50 chips especially for the case you're describing.