Is it permissible to change the intention of sacrificing a goat if the medical tests are reassuring, and to spend its value in cash on needy relatives instead of sacrificing it?

If you said, "I will slaughter a kid if the test results are reassuring," and you did not intend it as a vow (nadhr), then nothing is binding upon you. However, if you did intend it as a vow, then it becomes binding. But if you did not utter words indicating a vow and merely thought it to yourself, then the vow does not become binding.

If you uttered these words with the intention of making a vow, and the test result was reassuring as you intended, then the vow becomes binding upon you, which is the slaughtering of the kid, and it is not permissible to substitute it with money. You may distribute the kid among your needy relatives upon whom you are not obligated to spend.