All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
You incur no sin by failing to fast on that day because you did not hold the intention of fasting. The fact that you made the fasting of that day conditional on waking up to have the Sahoor (pre-down meal) does not serve as an intention of fasting, without which the fasting is invalid. This is because intention should not be made conditional upon anything; it should be firmly held with no room for doubt or hesitation.
Al-Mawsoo‘ah Al-Fiqhiyyah Al-Kuwaitiyyah (Kuwaiti Fiqh Encyclopedia) reads: "Muslim jurists unanimously agreed that intention must be held with firm determination and resolve because it is a condition of the validity of worship acts. The Prophet, sallallaahu ‘alayhi wa sallam, said: “Verily, deeds are judged according to the intentions…” Intention is defined as, “The firm resolve in the heart to do something, i.e. to have the resolution in the heart to do something, and to be determined to do it without hesitation." It is not a mere willingness or inclination to doing something. Rather, it has to be held with resolve with no hesitation or conditioning that undermines such determination. When a person makes his intention conditional on the Will of Allaah, then if he actually intends this (such as to say that one will do such-and-such if Allaah wills otherwise no) or that he makes an unspecified conditional, the intention is invalid because of the lack of resolve. However, if one makes one's intention conditional on the Will of Allaah, seeking thereby the blessing of Allaah, the intention is valid. Hesitation and conditioning one’s intention on the occurrence of something, even if it is unlikely to take place, renders it invalid.”
For further information, please, refer to Fatwa 89096.
Allaah Knows best.