Does one owe repentance and expiation for swearing to abandon masturbation for a month, then rubbing his penis and feeling a fluid, not knowing whether it was semen or pre-seminal fluid?

Masturbation is forbidden, and repentance for it is obligatory. The reference point for oaths is the intention of the one who swears it. If you intended to abstain from experiencing pleasure in this manner, even if semen was not discharged, then you have broken your oath and atonement is incumbent upon you. If, however, you intended not to discharge semen through masturbation, and did not intend to abstain from merely rubbing the private parts or similar actions, then you have not broken your oath unless you are certain that semen was discharged.