If you think about it, on Sasuke's travels he could have produced Salada with Karin. Around the time of birth or exactly at birth, Karin could have died. Sasuke still not done with his rectification walk understood Sakura would get impatient or maybe he thought she was lonely so he gave the baby to her to raise and Sakura being as unconditional lovingly as she is towards Sasuke, raises Salada as her own. Just to drop a theory