My neighbour has encroached my site

In 2020 I sold two plots as 30x80 ft. However, the actual measured dimension was 30x79 ft, with the one-foot difference attributed to a drainage line constructed around the layout. The original buyers acknowledged and accepted this difference verbally, understanding its purpose, but this agreement was not documented in writing. Recently, one of these original buyers sold a portion of their land, specifically 30x39 ft, to a new individual. This new buyer has now encroached upon our adjacent property. Their justification for this encroachment is an alteration of the plot dimensions to 31x39 ft. They claim this increase in the East-West dimension (from 30 to 31 ft) is to compensate for a one-foot reduction in the North-South dimension, which they measure as 39 ft. It's crucial to note that the original documented dimensions for this portion of land were 30x40 ft (totaling 1200 sq ft), whereas the new buyer is now claiming 31x39 ft (totaling 1209 sq ft). Does buyers have an option to encroach and compensate their difference?? What should be my next step