To protect the stock price, a bank will place 10 million euros worth of the shares every month until no more are left.
"The sale will be carried out at the bank's own discretion in the stock market or over the counter, for the first time in November 2012 and then again in the months January through November 2013," SAP said in a regulatory statement.
SAP shares reversed their gains, trading down 0.5 percent at 59.91 euros by 1554 GMT.
Plattner is SAP's largest shareholder with 121.49 million shares, representing a 9.89 percent stake, according to Thomson Reuters data from the end of September. Based on the current share price the stake is worth a little over 7 billion euros.
With a 73 billion euro market capitalisation, SAP is the most valuable company in the German blue chip index, followed by Siemens, worth a little under 70 billion.
© Thomson Reuters 2012