ALL the above suggestions will most likely work, but here is an easy and inexpensive fix:
You will need an ATV, a spikey thing, a water source with a high pressure nozzle and a dragging screen.
Get a high pressure water nozzle and from the outfield, and facing toward the infield use the high pressure water to blow the excess dirt (the lip) toward the infield, and do ALL the perimenter of the infield. This takes an hour or so depending on the size of the lip.
Next allow the excess water overnight or so to drain into the ground/infield dirt. Next, using the ATV get one of those spikey grader tools (I made one out of steel tubing and welded spikes about 2" tall on the end) and this will turn over the infield and more importantly "fluff" up the infield dirt/clay. After the infield dirt/clay is fluffed up I flipped over the spikey tool and utilized the tool as a "leveling" device. As you circle the infield, drag the excess dirt to the low spots and there will be low spots.
After the infield is as level as possible, use the screen dragger behind the ATV and groom the infield to a desirable finish.
NOTE: Make sure when using the spikey tool, to use a danger cone/delineator, bucket or something that will divert you from spiking the base pegs, home plate and of course the pitchers rubber.
All this is accomplished without buying more dirt.
Good luck!!