I understand that the a fixed time for Torah takes precedence over an even large monetary gain. I am also aware of the general rule that if a Mitzvah cannot be performed by someone else, then it comes before learning Torah. Dose a this rule of "mitzvah she’i efshar la’saos al ydei acher" apply even during one’s fixed time to learn Torah, or does this time have a special status, so one must continue learning? Thank you very much in aadvance.

In general, the time allotted for Torah study should be considered Holy-of Holies, nevertheless, if halachically a particular mitzva supersedes Talmud Torah, like in the example which you gave, than it does so even during one's allotted time for Torah. Try to make-up the Torah time afterwards.