Shabbat 72 - Guilt
A woman slave may be designated for another male slave, and in that case she is prohibited to other men. However, the punishments for cohabiting with her is different from the standard one of a regular married woman, and consists of bringing a guilt offering. Moreover, for multiple acts of cohabitation he is still liable for only one offering. This stands in contrast to Shabbat violation, where one brings an offering for each violation .
Yet another leniency is that one brings the same offering whether one cohabits with a woman slave by mistake or intentionally. And, as Ulla stated, even if one brings a guilt offering without being aware that he brings it for his cohabitation with a woman slave, it is still effective.
Rav Hamnuna challenged Ulla: "Imagine that one cohabited with a woman slave, then cohabited again, then designated and brought a sacrifice to the Temple, and now he tells the Kohen to wait a little longer till he cohabits again and comes back, so that his sacrifice will atone for the newest act also, will it work? Do you mean to go that far?" To this Ulla replied, "You are talking about an act after the designations of the sacrifice? That I never have meant; after designation he will need another offering."
Art: William James Muller - The Slave Market