$username = userdomainusername
$password = cat C:securestring.txt | convertto-securestring
$cred = new-object -typename System.Management.Automation.PSCredential -argumentlist $username, $password
Invoke-Command -ComputerName server_b -ScriptBlock {sqlcmd -S server_bsqlexpress -E -d ReleaseTeam -Q "exit(exec[dbo].[SP_NAME])"} -credential $cred

Copy-Item "\server_asharenamefile.ext" "\server_bsharename"

Copy-Item : The network path was not found.
At line:1 char:10
+ Copy-Item <<<<  "\server_asharenamefile.ext" "\server_bsharename"
    + CategoryInfo          : NotSpecified: (:) [Copy-Item], IOException
    + FullyQualifiedErrorId : System.IO.IOException,Microsoft.PowerShell.Commands.CopyItemCommand

server_a True on server_b
server_a True on client
server_b True on server_b
server_b False on client

Invoke-Command -ComputerName server_b -ScriptBlock {test-path -path "\server_asharenamefile.ext"} -credential $cred