Praise be to Allah.
There is no sin on you for this delay, so long as it is due to you being in financial difficulty and working to accumulate enough money to cover the costs of getting married, because it is due to a valid excuse. But you should seek their pardon and tell them about the situation, and that you need some more time to accumulate what you need of money. If you tell them about that and they are okay with that, there is nothing wrong with that and there is no blame on you. What we advise people is not to burden the husband with costs of marriage that he cannot afford; rather they should be easygoing with regard to marriage and the mahr, and reduce the costs of marriage as much as possible. End quote.
And Allah knows best.
Majmoo‘ Fataawa ash-Shaykh Saalih al-Fawzaan (2/549).