WASHINGTON — Hundreds of thousands of federal employees returned to work on Monday with a presidential promise of a paycheck “very quickly” but no guarantee that they will be working in three weeks, when a temporary stopgap of funding expires.

President Trump and congressional Democrats arrived at a short-term agreement late on Friday, reopening the government after 35 days and the longest government shutdown in history. Lawmakers have until Feb. 15 to reach a compromise on the Republican request for billions of dollars to be allocated for a border wall — a wall Democrats have refused to fund.

Referring to the odds that a deal could be struck over that time, Mr. Trump told The Wall Street Journal on Sunday, “I personally think it’s less than 50-50.” Mr. Trump said he would use emergency powers to fund the wall if an agreement could not be reached.

In a stark reminder that federal employees were returning to work with the knowledge that they might be forced to go without a paycheck once again next month, one federal agency, the Department of Agriculture, updated its employee information website and said, “We will also leave some of the shutdown-related material up for a period time, should you need to refer back to it.”