obiecywać komuś gruszki na wierzbie
Idiom USED On Occasion BY Everybody can use it
(to promise someone pears on the willow) • When you promise something that is not possible / to make empty promises.
"Prezydent obiecywał gruszki na wierzbie, byleby tylko wygrać wybory."
"The president promised the pears on the willow to win the elections."