Consulting task for Witte

Topic - right to social security
Circumstances of the case: Hello! I need legal advice on the legality (illegality) of the actions of the social security agency.
I am a departmental pensioner. She retired in 2004. In the same year, she received a city social supplement to her pension (6,413.20 rubles).
In 2006 I got a job. Worked from 2006 to 2010.
From 2004 to the present time (without interruption) she has received a social supplement to her pension. In January 2019, I contacted the social welfare department with
application to recalculate the regional social supplement (RSD) for pensions to the value of the city social standard of 17,500 rubles. for 2018 + January 2019 because in total, the pension and additional payment did not correspond to the city social standard (pension 9765.50, social supplement 6413.20 rubles).
I received a recalculation of the RSD from February 2019.
- From February to March 2019, I received the city social standard of 17500 (pension 9765.50 +7734.50 - social supplement).
When I asked why they didn’t recalculate my social supplement to the city standard for 2018, I was told that the underpayment would go towards paying off the debt for 2010, when I worked and received the supplement.
- For May 2019, I received 9765.50 + 6187.60 i.e. 15953.10 rub.
When I asked why the amount paid did not correspond to the city social standard, the social protection department explained to me that from May they would deduct 20% of the amount of the additional payment for 2010, when I worked and received an additional payment towards my pension.
I also received a letter from social security with a protocol for collecting 20% and calculation of the overpayment.
Excerpt from the letter: On 03/22/2019, as part of interdepartmental interaction, the department, when checking information in the process of providing a state function, received information from the pension fund in the SIZ-5 form about the period of work from 06/01/2006 to 05/31/2010.
THE DEPARTMENT DECIDED:
1- the excess amount of regional social supplement paid due to the fault of the pensioner during the period of work from 01/01/2010 to 05/31/2010 is subject to recovery
2- Withhold the excess amount of regional social supplement received monthly at 20% until the debt is fully repaid.
It turns out that they did not recalculate the RSD to the city standard for 2018 and began to deduct 20% of the amount of social supplement from May 2019.
Based on the above, I ask you to advise on the following issues:
- is there a statute of limitations for collecting social security debt (9 years have passed)?
- Is it legal that they did not recalculate the RSD for 2018 to the value of the city social standard of 17500 (9765.50 pension, 6413.20 additional payment)?
- if there are violations of my rights by the social protection department, then what should I do?