Ryathal's answer is correct, but I'll go into a bit more detail:
Article One, Section 6, Clause 2 of the Constitution, aka the Emoluments Clause or Ineligibility Clause says:
No Senator or Representative shall, during the Time for which he was elected, be appointed to any civil Office under the Authority of the United States, which shall have been created, or the Emoluments whereof shall have been increased during such time; and no Person holding any Office under the United States, shall be a Member of either House during his Continuance in Office.
In other words:
- If a new Office is created while you are in the Senate or the House, you are ineligible to take that position for the duration of your elected term. Likewise if the compensation is increased. (see the Saxbe fix for a workaround to the latter)
- If you are holding an Office, you must resign your seat in Congress (or vice versa).
However, note the phrase
"be appointed to". This means that elected offices, such as the presidency or vice presidency are not covered by this clause. So Ford's and FDR's salaries would not have been affected by this.