The Spanish club also has the option to buy the 26-year-old forward, who made just 15 appearances after signing for Brendan Rodgers's side from Celta Vigo last summer
The Spanish club also has an option to buy the forward, while the move is subject to a medical and the relevant paperwork being completed.
The 26-year-old joined Brendan Rodgers' side from Celta Vigo last June but made just 15 appearances in all competitions last season.
Aspas' only goal during his debut campaign at Anfield came in the 2-0 win over Oldham Athletic in the FA Cup third round.
Prior to Aspas' exit, Rodgers bolstered his attacking options last month with the signing of Rickie Lambert from Southampton, although Luis Suarez is on the verge of completing his 94 million euro move to Barcelona.
Liverpool is also expected to complete a 25 million euro deal for Benfica forward Lazar Markovic. The 20-year-old already has arrived on Merseyside to undergo a medical and finalize personal terms.