I just had this same scenario in August. My niece asked for a quilt for her wedding, a king size. Unfortunately, we had some major home expenses; New HVAC and a tax bill that had to be paid prior to the wedding date. We could not afford to buy all the supplies to have the quilt done. We gave a card, enclosing a small check and a note that they would be getting their quilt before their first anniversary. The bride and groom understood perfectly and it also gave me the opportunity to see what things they used in their wedding theme - butterflies, sunflowers, and arrows - that I could incorporate into the quilting designs to make the quilt more personal and special for them.
Don't sweat it.