Which is better: to say "may it be in the balance of your good deeds" or "may it be in the balance of your good deeds – if Allah wills"? – taking into consideration the issue of conditioning supplication upon Allah's will.

If the intention is to supplicate, then it should not be conditioned upon Allah's will. Rather, one should say: "May Allah make it (or place it) in the balance of your good deeds," based on the Prophet's (peace and blessings be upon him) saying: "None of you should say: 'O Allah, forgive me if You will,' or 'O Allah, have mercy on me if You will.' Rather, let him be firm in his request, for Allah cannot be compelled." However, if the intention is to inform, then it should be conditioned upon Allah's will, because knowledge of that matter rests with Allah.