Individual Missionary Donation

If I donate money to a specific missionary from a ward, does that money go directly to his account for him to be able to use on his mission?

The language at the bottom of the donation slip would seem to shed light on the question:

Though reasonable efforts will be made to use donations as designated, all donations become the Church's property and will be used at the Church's sole discretion to further the Church's overall mission.

If I understand correctly, with the pooled missionary funding system that has been in place for quite a few years now, the proper amount of money is always available at the proper time for the missionary to use, so there is no worry about a missionary going hungry if perchance nobody donated to his/her account. The ward is expected to provide enough funds for the total number of missionaries serving from that ward. That total account can go positive or negative. I'm not acquainted with current details of accounting procedures for individual missionaries.

Did that help answer at least part of the question?

No. Missionaries are given a specific amount each month for their expenses based on the expected cost of living in the area where they are serving. This amount is not related to the amount the missionary's ward is charged each month for the missionary. When you donate money to a specific missionary, you are basically helping the ward fulfill it's commitment for the missionary. You are reducing the amount the family needs to provide in order to meet their commitment. It does not give the missionary more money to spend.

IRS has specific rules about tax deductions for charitable donations. In 1984, the Supreme Court ruled that charitable donations for missionary work are tax deductible as long as the missionary work is a direct furtherance of the Church's or charitable organization's charter. The Supreme Court further clarified in about 1992 that donations directed only to specific missionaries are not tax-deductible but are taxed as wages. To "close the loop" on this, the Church since about 1994 treats all missionary donations it receives, including those to General Missionary Fund, those to Ward Mission Fund, and those directed to individual missionaries via tithing slips, as Church property and are not refundable. You will receive a tax statement at the end of the calendar year that lists your missionary contributions as "charitable donations." The MLS payments system records donations directed to a specific missionary as a credit to that missionary's entire mission obligation (currently $500 per month for missionaries from U.S.), but that specific missionary will not directly receive your extra funds that month--all missionary donations are deposited into a central missionary account and disbursed via centralized controls to the missions and missionaries per routine every month. If a missionary needs funds for extra items while on their mission, the extra funds are typically sent directly to the missionary by friends/family/ward members (but not from the ward or general account), and are then used only by that missionary for those specific personal needs and are not tax-deductible per the IRS rules above. However, each missionary's situation is unique.