[GRASS-user] buffer vector within a given area

[Maris Nartiss]

If I got it right - You need to fill possible agriculture land till it
reaches required area.
I think You can use mapcalc sample provided in r.lake documentation.
Just replace ${variables} with Your map name and calculate resulting
map area after every run. Mask nonagriculture land with MASK and it
will not be used.
Like (pseudocode):
while $area<$target:
 r.mapcalc newmap={code from r.lake docs}
 area = r.report newmap area

If You need additional help, drop a note.
Maris.