From the excellent ECJblog, I discover a fascinating case about the principles of restitution in EU law (emphasis added):

Case C-470/08, Kornelis van Dijk v Kampen

Since 1982, Mr van Dijk had leased from the Dutch Municipality of Kampen (pictured right) a number of parcels of agricultural land … The lease between the two parties did not contain any clauses relating to the income supported scheme or payment entitlements.

For a number of years Mr van Dijk had received … [various EU] compensatory payments … A dispute arose between Mr van Dijk and the Gemeente Kampen regarding the nature and the extent of the obligations under the lease. The referring court essentially asked whether Community law required a lessee, on the expiry of the lease, to deliver to the lessor the leased land along with the payment entitlements accumulated thereon or relating thereto, or to pay him compensation.

The Court held that … payment entitlements remained with the lessee on the expiry of the lease … [the relevant schemes] did not contain any obligation on farmers who had leased land to transfer their payment entitlements to the lessor on the expiry of the lease.

The Court held that, in accordance with the principles common to the laws of the Member States, the right to restitution from the person enriched was conditional upon there being no valid legal basis for the enrichment at issue (Case C-47/07 P Masdar (UK) v Commission [2008]).

It could not be considered that the payment entitlements which a farmer enjoyed were devoid of any legal basis in so far as they were attributed to him in accordance with the provisions of [a] Regulation … Therefore, the Court concluded that Community law did not require a lessee, on the expiry of the lease, to deliver to the lessor the leased land, including the payment entitlements accumulated thereon or relating thereto, or to pay him compensation.