First thing to note is that by this point in the story, Moss had already removed the transponder. It happened right before he and Anton had their confrontation that led to both getting injured. That meant that only Moss and Wells knew where the case was. Moss was in the hospital, so Wells had a comfortable advantage in retrieving it.

That's the core idea in his decision. There was no rush. There was no need to climb over a hard-to-climb fence in broad daylight in a suit, risking an injury and drawing attention. There was no need to immediately go trekking around in tall grass. Wells could wait for night, get backup, change into something more appropriate, and still likely be the one to retrieve the case.

That said, I don't believe it's ever explicitly stated what his plans were. He just clearly did not want to climb the fence.