The money will go towards aid workers on the ground in Syria through various UN agencies.
The truce began at midday (09:00 GMT) on Sunday, in the provinces of Deraa, Suweida and Quneitra in the southwest, along the Jordan border.
A monitoring group said there had been no air strikes or clashes since the ceasefire started. The UK-based Syrian Observatory for Human Rights said “calm was prevailing” in the area.
A rebel official in Deraa city, speaking to Reuters news agency, also said there had been no significant fighting.
Al Jazeera’s Natasha Ghoneim, reporting from the Turkey-Syria border town of Gaziantep, said the focus on the southern region was down to worries by US allies in the region.
“This is an example of foreign interest at play and the interest of US allies,” Ghoneim said.
“The southwestern portion of Syria is a very geopolitically sensitive area. To the west you have Israel and to the south you have Jordan. Both of those countries have been very worried about violence spilling over their borders.”
Similar agreements have been brokered in Syria in the past with the aim of getting the country’s peace process back on track following a prolonged civil war that began in 2011.
All have failed to halt the fighting for very long.
But in a statement issued late on Saturday, US National Security Adviser HR McMaster said the US was “encouraged by the progress made to reach this agreement.
“The United States remains committed to defeating ISIS [ISIL], helping to end the conflict in Syria, reducing suffering, and enabling people to return to their homes,” he added. “This agreement is an important step toward these common goals.”
Syrian government forces had already announced a halt to combat operations in the three provinces.
The deal is separate to the four so-called de-escalation zones, which are being negotiated by Russia, Turkey and Iran.
Those talks broke down in Kazakhstan’s capital Astana last week, over the policing and precise borders of the areas.
Syrian rivals are to come together for a seventh time in Geneva in the coming days.
Diplomats from Russia, Iran, Turkey and the US were participating in the Astana talks along with UN special envoy Staffan de Mistura.