Is it obligatory to commit to sacrificing a calf worth ten thousand pounds upon the birth of a daughter, or is it permissible to donate the amount to a more deserving cause of charity?

If you did not verbally declare the vow, then you are not bound by it, and you may spend the equivalent amount on what you deem more deserving among charitable causes. However, if you intended the vow, then it is considered a valid vow established by implication alongside the intention, and you must fulfill it in the manner you mentioned. It is not permissible for you to retract it or substitute it.