I want to show a Christian friend of mine that it is important to keep one's word once it's given. Is there anything in the Bible to help me ?
God takes our promises seriously ...
When thou vowest a vow unto God, defer not to pay it; for he hath no pleasure in fools: pay that which thou hast vowed.
... so much so that ...
Better is it that thou shouldest not vow, than that thou shouldest vow and not pay.
The ideal is to have a such a character of faithfulness and honesty that you would not need to swear at all, but people would just take you at your word:
But I say unto you, Swear not at all; neither by heaven; for it is God's throne: Neither shalt thou swear by thy head, because thou canst not make one hair white or black.
In Jam. 5:12, it is written,
But above all, my brothers, do not swear, neither by the heaven, neither by the earth, neither by any other oath, but let your "yes" be "yes" and your "no" be "no," lest you fall into condemnation.
πρὸ πάντων δέ ἀδελφοί μου μὴ ὀμνύετε μήτε τὸν οὐρανὸν μήτε τὴν γῆν μήτε ἄλλον τινὰ ὅρκον ἤτω δὲ ὑμῶν τὸ ναὶ ναὶ καὶ τὸ οὒ οὔ ἵνα μὴ εἴς ὑπὸκρίσιν πέσητε