Must 4,0 be given as charity from the first salary, even though the amount received is less than expected, or can it be given from the second month's salary?

Oaths and vows are referred to the intention of the swearer or vower. If there is no intention, then to the reason for the oath or vow. Otherwise, to the apparent meaning of the word.

If your intention was to donate four thousand from the first complete salary, then you are not obligated to fulfill it except from the full salary. If you had no intention, then the four thousand became binding from the first salary you received, even if it was incomplete.

The principle is that "in oaths, reference is made to the intention of the swearer if the wording allows for it." If the intention is absent, "reference is made to the reason for the oath and what prompted it." If that is absent, "reference is made to the specific designation." If all of that is absent, "reference is made to what the name encompasses."