Why are Senegal’s delayed elections controversial?

Senegelese President Mackay Sall, shocked the country over the weekend by announcing that the upcoming February 26th election would be delayed.  This led to protests and a vote by lawmakers, agreeing to extend the Presidents tenure and delay elections until December. Is this move lawful? Also, Botswana often tops the list as Africa's least corrupt country, so what is it doing differently?And how did former president Nelson Mandela’s personal belongings end up in an auction?