Can fundraiser money be used for supporting a missionary?

Stake clerk here. We have a unit who did a fundraiser for scout camp this year and obtained more money than was needed. A question was raised whether this excess money can be used to help pay for missionaries in the ward. Is this allowed? Thanks.

No, funds that are raised must be placed in an Other account for a specific purpose and then used for that purpose. See The "Other" Category article in the help center, particularly the first bullet point under "What Clerks Should Know". That article also states that funds collected but not used should be returned to the donors (in the second paragraph under Policy and Principles). But that's often difficult when the money comes from a fundraiser.

If there is any camp equipment the unit needs it can be purchased with the excess funds. Otherwise, the funds should either be retained for scout camp the following year (with either no fundraiser or a smaller one) or turned into CHQ as indicated at the end of the above article.

Thank you.

can you ask specific members to donate a specific amount towards a missionary

Information on missionary financing is found in the General Handbook section 24.3.4. It says that the bishop or stake president may ask members to contribute to the ward missionary if there is a need. It does not address whether they can ask specific members or for specific amounts. That would be something the bishop and stake president would have to decide.