First, scriptures may be interpreted in different ways, but all Scripture is not 100% open to interpretation. For example, when the Bible says "Jesus wept," one should not interpret that as meaning "Can't sleep, clowns will eat me." One must interpret the Scripture reasonably. So, here's some reasonable interpretation for you, mixed together with some common sense:
1. The Bible is silent on organ donation. Silent. Nothing there. In the Scripture you are referencing, Jesus was discussing the issue of fasting with the Pharisees. None of the Pharisees were organ donors, organ recipients, or surgeons who could accomplish the feat of organ transplant. Therefore, it is reasonable to assume that the Scripture does not apply to organ donation.
2. God has given man free will, so that a person is free to act outside of God's will. For example, murder is against God's will. Yet, people commit murders. Therefore, we can see that God allows things to happen outside of His will. Therefore, bodies rejecting transplanted organs is not evidence of organ donation being against God's will.
3. Some organs reject the new body. Not all do. Therefore, even if your logic was sound (which it is not), then only some organ donations are outside of God's will.