The Spanish club also have 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's side from Celta Vigo last June but made just 15 appearances in all competitions last term.
Aspas's 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's 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 £70 million move to Barcelona.
Liverpool are also expected to complete a £19.8m deal for Benfica forward Lazar Markovic. The 20-year-old has already arrived on Merseyside to undergo a medical and finalise personal terms.