...He didn't pay her. She is not a prostitute, she is a slave and he paid her owner. At no point she could have refused to have sex with him and she was obviously in distress - Tyrion noticed how dead her eyes looked and she was crying her eyes out when he forced himself on her the second time after throwing up on the expensive carpet (which she would be most likely beaten for). What he offered her doesn't matte at allr, because as you say yourself, she didn't even understand him and even if she could take him up on that offer (did she have a blade? would he really have let her kill him? could she have overpowered him if he started resisting?) she wouldn't make it past the door.