While I was volunteering at the J C Raulston Arboretum today I showed your pictures to the horticulturist there and he wasn’t sure if it was a fungus or sun scald, but as soon as he heard it came from Lowe’s his reaction was just take it back and get your money back.  Pussy willows do not do well in our heat so are short lived even when healthy, and a mass produced plant at Lowe’s may not have been well taken care of giving it less of a chance.  The leaf dieback does not look like a healthy plant.  I am going to assume that you looked at the plant carefully at the time of purchase and that all this happened in 3 weeks.  Lowe’s should happily refund your money.