MS SQL Server 2012... with nightly databases backups to the same/local as SQL Server machine... 

I am trying to add another SQL Server Agent Job to copy the *.bak* files to remote (non-windows, i.e. Linux) share with non-Windows (non-AD) user/password credentials. 
I do not have any access to configuring or changing that access which is under control of other, quite remote people. 

For this (copying) I created local user with the same user name and password, gave it permissions to the (source or local) backup-folders upon which all perfecly works from command line (`Win + К` or `cmd`) if to enter the command manually: 

```
RUNAS /user:UserName /savecred "robocopy d:\SQLBACKUP  \\10.195.xx.yyy\backup /S /purge /MAXAGE:7 /MT:1 /Z"   

```

but fails to run as **SQL Server Agent** job (type of step is "`Operating System(CmdExec)`". SQL Service Agent (with standard configuration of running under `[NT Service/SQLServiceAgent]` account, the job is owned by `SA` SQL Server superuser). 

Can anybody explain me why it is failing and how to correctly make it running (taking into account that I do not have access to domain users configuration)?