It becomes a
Sadaqa Naafilah (Optional Saddaqa)
instead since it is not obligatory.
Zakat is considered Saddaqa Waajibah (Obligatory Saddaqa)
There is no limit (minimum or maximum) to how much Saddaqa Naafilah you can give.
The amount of blessings one gets are only known to Allah.
As an example, there is the case of Hazrat Abu Bakr and Hazrat Umar. Donations were being sought to fight the Tabuk expedition. Reference
Hazrat Umar Farooq decided that he would give exactly half of
everything he owned. Once he collected it all, he took it to
Rasulallah , who then asked him what he had left for his family.
Hazrat Umar replied that he had left exactly half of his wealth for
his family, and Rasulallah accepted his donation.Shortly afterwards,
Hazrat Abu Bakr was on his way to see Rasulallah and was carrying a
small package with him which contained everything he possessed. He
then gave it to Rasulallah , who asked him what he had left for his
family. Hazrat Abu Bakr Siddique replied that he had left Allah and
his Beloved Rasul for his family.
At this Rasulallah smiled, and it was then that Hazrat Umar Farooq
admitted that he could never surpass Hazrat Abu Bakr Siddique in
willingness to sacrifice for Allah's cause.
So even though Hazrat Abu Bakr's donation was smaller, but it carried greater favor because as a percentage of his wealth, it was greater.