I suggest we make a slight change to the way the player's take from girls' activities is handled. Currently, the game displays how much the girl was paid and what the player's take of that total is. The problem with this system is that it makes detecting thieves trivial: if the amount the player receives < income * house take, then the girl is stealing (allowing some small room for rounding errors). I suggest the following:
1) Only display what the customer pays the girl if you have goons assigned to watching the girls, and only then if they happen to observe the transaction take place.
2) Girls should be more inclined to keep the tips they receive from customers, rather than handing them over to the player.
3) Slave girls should have a chance of keeping their tips and stealing from the player as well, though it might be lower than free girls.
Alternatively, when we get the girls-assigned-as-matrons implemented, all transactions could go through the matron. Customers pay the matron for services, and then tip the girls at their discretion. The matron gives the girls their cut and passes the profits on to the player, but she might skim off the top, "forgetting" to record some transactions. Gangs assigned to spy duty might notice something fishy going on; ambitious girls might report the matron's theft, hoping for a promotion.