where i am wrong suggest me alternative

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ASP.NET

//this code is used when employee is applying for Claim(emp module code)

using (SqlCommand _oCmd = new SqlCommand())
            {
                _oCmd.Connection = _oCon;
                _oCmd.CommandType = CommandType.StoredProcedure;
                _oCmd.CommandText = "dbo.stp_iOpdClaim";
              _oCmd.Parameters.Add(new SqlParameter("@userid", lbldisplay.Text.Trim().ToString()));
                _oCmd.Parameters.Add(new SqlParameter               ("@claimamt",txttotalcost.Text.Trim().ToString()));

                _oCmd.Parameters.Add(new SqlParameter("@status", "pending"));
                _oCmd.Parameters.Add(new SqlParameter("@Balance",15000));
                       
                _oCmd.ExecuteNonQuery();
                ClearControls();
                GetSaveMessage();
            }
        }

emp can claim upto 15000 that's why 15000 is store in Balance

//admin module code for approving/reject claim

from 1ST claim suppose i apply for 5000 claim amt then admin will verify it and approve it so in database 10000 is save for 1st claim whenver i claim from 2nd timeclaim amt for 2nd claim will subtract from 10000 but it was subtracted from 15000 only plz ckeck code where i am wrong suggest me alternative

_sr = Request.QueryString["sr"].ToString();

 protected void SaveDetail()
    {
        
        DataSet ds = new DataSet();
        DB obj = new DB();
        string str = "update tbl_OpdClaim set Balance =" + txtbalance.Text + " ,status='" + txtstatus.Text + "' where userid='" + _sr.ToString() + " ; 
    
        obj.execute_dml(str);
        GetSaveMessage();
        obj.close();
       

    }

 protected void cmdverify_Click(object sender, EventArgs e)
    {
        DataSet ds = new DataSet();
                                
        DB obj = new DB();
    
        
   string str = "select totalamt,Balance from tbl_OpdClaim where userid='" + _sr.ToString() + "'";
                 ds = obj.getdataset(str);
        
       double  bal = Convert.ToDouble(ds.Tables[0].Rows[0]["Balance"].ToString());
        double  claimamt = Convert.ToDouble(ds.Tables[0].Rows[0]["totalamt"].ToString());
         if (claimamt < bal)
             bal = bal - claimamt;
         txtbalance.Text = bal.ToString();
                  
    }
    protected void rdbStatuschange_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (rdbStatuschange.SelectedValue == "Approved")
        {
            txtstatus.Text = "Approved";
                       
        }
        else
        {
            txtstatus.Text = "Rejected";
        }
    }